The present invention relates to a cathode ray tube, and particularly to a color cathode ray tube having an in-line type electron gun configured so as to project three electron beams arranged horizontally in a line toward a phosphor screen.
Cathode ray tubes for use in TV receiver sets or information terminal display monitors have at least an electron gun comprised of plural electrodes and a phosphor screen and is provided with a deflection device for scanning plural electron beams emitted from the electron gun on the phosphor screen.
For these cathode ray tubes, the following technologies have been known for reproducing a good image over the entire phosphor screen.
An electrostatic quadrupole lens is formed of electrodes in an electron gun, and the strength of the electrostatic quadrupole lens is varied dynamically with deflection of the electron beam to obtain uniform display image over the phosphor screen as disclosed in Japanese Patent Application Laid-Open No. Sho 61-250933 (Application No. Sho 60-90830, laid-open on Nov. 8, 1986), for example.
FIG. 11 is a schematic plan view of a color cathode ray tube having a electron gun employing the prior art electrostatic quadrupole lens. Reference numeral 1 denotes a glass envelope, 2 is a faceplate portion, 3 is a phosphor screen for displaying an image, 4 is a shadow mask, 5 is an internal conductive coating, 6, 7 and 8 are cathodes, 9 is a first grid electrode (a beam control grid electrode or G1 electrode, hereinafter grid electrodes are abbreviated as G electrodes), 10 is a G2 electrode (an accelerating electrode), 11 is a G3 electrode, 12 is a G4 electrode, 13 is a first G5 sub-electrode, 14 is a vertical electrode piece, 15 is a horizontal electrode piece, 16 is a second G5 sub-electrode, 17 is a G6 electrode (an anode), 18 is a shield cup, 19 is a deflection yoke (a deflection device), 20, 21 and 22 are center axes of the respective cathodes 6, 7 and 8, 23 and 24 are center axes of respective outer apertures in the G6 electrode 17. In FIG. 11, the phosphor screen 3 comprising alternate lines of three color emitting phosphors is coated on the inner surface of the faceplate portion 2 of the glass envelope 1.
The center axes 20, 21, 22 of the cathodes 6, 7, 8 are aligned with those of apertures corresponding to the respective cathodes in the G1 electrode 9, the G2 electrode 10, the G3 electrode 11, the G4 electrode 12 for forming a pre-main lens in cooperation with the G3 electrode 11, the first G5 sub-electrode 13 and the second G5 sub-electrode 16 of the focus electrode serving as one lens component of a main lens and the shield cup 18, and arranged approximately parallel with each other in a common horizontal plane.
The center axis of the center aperture in the G6 electrode 17 serving as the other lens component, an anode, of the main lens is aligned with the center axis 21, but the center axes 23, 24 of two outer apertures in the G6 electrode 17 are displaced slightly outwardly with respect to the corresponding center axes 20, 22 in the common horizontal plane.
The four vertical electrode pieces 14 are attached to the end of the first G5 sub-electrode 13 which is one on the cathode side of the two G5 sub-electrodes into which the focus electrode is divided such that the four vertical electrode pieces 14 sandwich the respective apertures in the end of the first G5 sub-electrodes 13 horizontally.
A pair of horizontal electrode pieces 15 are attached to the end of the second G5 sub-electrode 16 on the first G5 sub-electrode 13 side thereof such that the horizontal electrode pieces 15 sandwich three apertures in the end of the second G5 sub-electrode 16 vertically. These electrode pieces 14, 15 form an electrostatic quadrupole lens therebetween.
A plurality (usually three) of electron beams emitted from the cathodes 6, 7, 8 enter the main lens along the center axes 20, 21, 22 of the corresponding cathodes. The second G5 sub-electrode 16 serving as the focusing electrode is supplied with a focus voltage of about 5 kV to about 10 kV, the G6 electrode 17 serving as the anode is supplied with an accelerating voltage of about 20 to about 30 kV, and the G6 electrode 17 is at the same potential with the shield cup 18 and the internal conductive coating 5 coated on the inner surface of the glass envelope 1.
The center apertures in the first and second G5 sub-electrodes 13, 16 of the focusing electrode and the G6 electrode 17 are coaxial with each other and aligned with the center axis 21, and consequently the main lens in the center is axially-symmetrical, the center electron beam travels straight along the center axis after being focused by the main lens.
The center axes of the two outer apertures in the end of the G6 electrode 17 facing the second G5 sub-electrode 16 are displaced horizontally outwardly with respect to those of the two outer apertures in the second G5 sub-electrode 16 and non-axially symmetrical main lenses are formed in the paths of the two outer electron beams.
The outer electron beams traverse a portion displaced toward the center electron beam from the lens axis in a diverging lens formed in the G6 electrode 17 (the anode) side portion of the main lens region and receive a focusing action by the main lens and a force converging the outer electron beams toward the center electron beam at the same time. The three electron beams are converged at a point on the shadow mask 4. This convergence of three electron beams at the central portion of the phosphor screen is called static convergence (hereinafter abbreviated to "STC").
The three electron beams are subjected to color selection by the shadow mask 4 such that portions of each electron beam passed by the apertures in the shadow mask 4 excite only phosphor elements of its corresponding color on the phosphor screen 3 to luminescence.
The deflection yoke 19 for scanning the electron beams on the phosphor screen 3 is mounted around the funnel portion 32 for connecting the faceplate 2 and the neck portion 31 housing the electron gun. The deflection yoke 19 for use in color cathode ray tubes for monitors of information terminals employs a so-called saddle-saddle type deflection yoke having horizontal and vertical deflection windings wound in a saddle configuration so as to prevent leakage of magnetic fields from the monitor sets.
It is known that the three electron beams are converged at all points of the phosphor screen when the three electron beams are initially converged at the center of the phosphor screen, by combination of a so-called in-line type electron gun having initially three electron beam paths in a horizontal plane and a so-called self-converging deflection yoke generating specific non-homogeneous magnetic fields.
In general, there is a problem with the self-converging deflection yoke in that resolution at the periphery of the screen is degraded due to deflection defocusing increased by its non-homogeneous magnetic fields.
To solve this problem, the electrostatic quadrupole lens is employed. The first G5 sub-electrode 13 is supplied with a fixed focus voltage Vf, and the second G5 sub-electrode 16 is supplied with the fixed focus Vf superposed with a dynamic voltage dVf synchronized with deflection currents supplied to the deflection yoke.
With increase in deflection of the electron beams, the voltage difference between the first and second G5 sub-electrodes 13 and 16 increases and the lens strength of the electrostatic quadrupole lens formed by the vertical and horizontal electrode pieces 14 and 15 increases and provides a greatly astigmatic shape to the electron beam spots.
When the potential of the second G5 sub-electrode 16 is higher than that of the first G5 sub-electrode 13, the astigmatism produced is such that an intense core of an electron beam spot is elongated vertically and a low intensity halo of the electron beam spot is elongated horizontally to cancel the astigmatism introduced by the deflection of the electron beam and to improve resolution at the periphery of the screen. When the electron beam is not deflected, by making the potential of the first G5 sub-electrode 13 equal to that of the second G5 sub-electrode 16 to eliminate the non-axially symmetrical lens, the astigmatism is not produced and resolution does not deteriorate at the center of the screen.
In cathode ray tubes of this type, the distance between the main lens and the periphery (corners) of the screen is longer than that between the main lens and the center of the screen, the beam focusing condition at the center of the screen differs from that at the periphery of the screen, and there is a problem in that, if an electron beam is focused for the best at the center of the screen, the electron beam is defocused at the periphery of the screen, and the resolution is degraded at the periphery of the screen.
But in the electron guns employing the electrostatic quadrupole lens, when the electron beam is deflected toward the periphery of the screen, the potential of the second G5 sub-electrode 16 is increased, the potential difference between the second G5 sub-electrode 16 and the anode is decreased and the strength of the main lens is weakened.
Therefore the beam focus point (the image point) is moved toward the phosphor screen 3, the electron beam can be focused on the screen at its periphery also and deterioration in resolution at the screen periphery is prevented. Curvature of the image field as well as astigmatism can be dynamically corrected.
When the prior art is applied to a color cathode ray tube having a maximum diagonal deflection angle of more than 90 degrees, for example, the axial length of which is shortened by increasing its deflection angle for use in information terminal display monitors and the like, a required dynamic voltage becomes too high for use in monitors. If the dynamic voltage becomes high, transistors serving as drivers in the dynamic voltage circuit unit have to withstand greatly higher voltages, presently-used dynamic voltage circuit cannot be used without design changes, and the cathode ray tube cannot be replaced separately from the monitor set.